1. Field of the Invention
The present invention relates generally to a power measurement means of a mobile station for measuring power of pilot channel signals transmitted from base stations in an asynchronous code division multiple access (CDMA) communication system. More particularly, the present invention relates to a power measurement means of a mobile station for measuring transmission power of each base station using transmission diversity techniques when the mobile station receives pilot channel signals combined with a plurality of spread pilot channel signals that are separately transmitted through antennas of each base station after pilot channel symbols are spread by a scramble code which is allotted for each base station, and a plurality of antenna symbol patterns.
2. Description of the Related Art
A variety of multiple access communication systems have used a single antenna for data transmission between base stations and mobile stations. In these cases, there has developed a critical problem concerning the deterioration of quality of the communication causing damage to several data groups if transmission channels encounter fading. The recited problem has been solved by the use of transmission diversity techniques in which data or information are transferred through at least two antennas. That is, the communication quality of mobile communication systems due to the deleterious effects of fading may have been improved by diversity techniques. In third generation mobile communication systems, transmission diversity techniques whereby base stations transmit the same data through two antennas has been recommended to maximize the capability of data transmission and receiving.
Third generation mobile communication systems suggested by Europe entities (hereinafter, called 3GPP) have also requested that base stations use diversity techniques to maximize the capability of data transmission/receiving. However, a mobile station can not use diversity techniques since it is physically too small to incorporate two antennas. In other words, even if the mobile station incorporates two antennas, the gain of diversity is too small because the distance between the two antennas is too short. Other disadvantages of a mobile station using diversity techniques are the size and manufacturing cost of the mobile station. In view of the above problems, generally base stations have only been able to use diversity techniques.
FIG. 1 shows a mobile communication system in which a mobile station 16 receives signals transmitted from base stations through antennas in the radio mobile communication system. As shown in FIG. 1, the mobile station 16 may receive signals transmitted from not only base station 13 in the cell 10 in which the mobile station exists, but also from neighboring base stations 14, 15 in cells 11 and 12, respectively. Generally, the signal strength transmitted from the neighboring base stations is weaker than that of base station 13 in the cell 10 in which the mobile station 16 exists. However, as the mobile station approaches the border of the neighboring base stations 14, 15 in cells 11 and 12, the weaker the signal strength transmitted from base station 13 in the cell 10 where the mobile station exists, the stronger the signal strength transmitted from the neighboring base stations. Therefore, the mobile station 16 must frequently measure the power of pilot signals transmitted from not only base station 13 where the mobile station is located, but also the neighboring base stations 14, 15, in order to determine the time to perform hand-over.
FIG. 2 illustrates the structure of antenna symbol patterns for discriminating antennas when a base station transmits the same pilot channel signals through two antennas. According to the specification of 3GPP, a frame of a physical channel consists of a plurality of time slots (e.g., 15 time slots) and has a specific time duration (e.g., 10 msec). Data of each time slot of the frame is spread over a plurality of chips (e.g., 256 chips) by a scramble code generated in a scramble code generator (not shown). Each pilot symbol is also spread over 256 chips by the scramble code. The spread 256 chips per antenna symbol are spread over a pilot channel signal by an antenna pattern symbol. The first antenna symbol pattern is composed of 15 time slots whose sign are all the same, while the second antenna symbol is composed of 15 time slots where two slots of each pair of slots has an opposite sign. In other words, slots 2, 3, 6, 7, 10, 11, 14 and 15 of the frame have an opposite sign comparing to the rest of the slots of the frame.
FIG. 3 illustrates that a base station using diversity techniques simultaneously transmits pilot channel signals spread by the antenna symbol pattern of FIG. 2 through two antennas. Referring to FIG. 3, unmodulated pilot symbols consisting of all 1's are spread by Orthogonal Variable Spreading Factor Code (OVSF code) in multiplier 301. The spread pilot symbols output from the multiplier 301 are spread over 256 chips per symbol by I/Q scramble codes generated by the scramble code generator 304 in multipliers 302 and 303. The I/Q scramble codes are a digital data sequence allotted for each base station. Each spread I/Q signal output from multipliers 302 and 303 is spread over the first pilot channel signal by multiplexing the first antenna symbol pattern provided by the first antenna symbol pattern generator 305 in multipliers 307 and 308, respectively. The spread I/Q signals output from each multipliers 302 and 303 are also spread over the second pilot channel signal by multiplexing the second antenna symbol pattern provided by the second antenna symbol pattern generator 308 in multipliers 309 and 310, respectively. The first pilot channel signal corresponding to the spread I/Q signals spread by the first antenna symbol pattern is combined in an adder 311, and is transmitted through the first antenna 313 after frequency modulating in a modulator (not shown). The second pilot channel signal corresponding to the spread I/Q signals spread by the second antenna symbol pattern is combined in a second adder 312, and is transmitted through the second antenna 314 after being frequency modulated in a modulator (not shown).
As described in the above, base stations simultaneously transmit the spread pilot channel signals spread respectively by two antenna symbol patterns through two antennas. Therefore, when a mobile station measures the power of pilot channel signals by using the method of IS-95, the mobile station must separate the received pilot channel signals into the first and second pilot channel signal. This is because the pilot channel signals received by the mobile station consist of the first pilot channel signal spread by the first antenna symbol pattern, and the second pilot channel signal spread by the second antenna symbol pattern. The mobile station accumulates the received pilot channel signals for at least a two-slot period in order to separate the first and second pilot channel signals. When measuring the power of the first pilot channel signal transmitted from the first antenna of a base station, a mobile station despreads the received pilot channel signal by every two slots of a frame, such as pair of slots —AA—of the first antenna symbol pattern in FIG. 2 so that the mobile station can eliminate the second pilot channel signal which is transmitted from the second antenna of a base station among the received pilot channel signals having the first and second pilot channel signals combined. This method, however, does not measure the power of the second pilot channel signal transmitted from the second antenna of the base station. Thus, even though the second antenna of the base station transmits the pilot channel signal having a higher power than the first antenna, the mobile station may make an wrong handoff decision based upon power of the first pilot channel signal transmitted from the first antenna.
As illustrated in FIG. 5, after receiving pilot channel signals which combine the first and second pilot channel signals that are spread in a base station, a mobile station may separate the first and second pilot channel signals from the received pilot channel signals by multiplying the first or second antenna symbol patterns and the received pilot channel signals. The I/Q pilot channel signals received by the mobile station are despread in multipliers 501 and 506 by signals generated in multipliers 502 and 507, in order to separate the first pilot channel signal from the despread pilot channel signals. The signals generated in multipliers 502 and 507 are obtained by multiplexing a scramble code generated in the scramble code generator 505 and the first antenna symbol pattern, which is generated in the first antenna symbol pattern generator 510 of the mobile station. The received I/Q pilot channel signals are also despread in multipliers 504 and 509 by signals generated in multipliers 503 and 508, in order to separate the second pilot channel signal from the despread pilot channel signals. The signals generated in multipliers 503 and 508 are obtained by multiplexing a scramble code generated in the scramble code generator 505, and the second antenna symbol pattern, which is generated in the second antenna symbol pattern generator 511 of the mobile station. Using the above method, the pilot channel signals received by the mobile terminal through an antenna are separated into the first and second pilot channel signals, respectively. Each separated first and second pilot channel signal is integrated at integrators 512, 513, 514, 515 at every two slots. The integrated value output from the integrators is multiplied at squarers 520, 521, 522, 523, and are exclusively combined at exclusive combiners 524, 525, respectively. That is, the power of each pilot channel signal transmitted from the two antennas of a base station is obtained. As a result, the power of each of the pilot channel signals of the two antennas are combined in an adder 526, and thus the mobile station may measure the whole power of the pilot channel signals of the base station regardless of the ratio of receiving power of each antenna at every two slots. This method, however, should have the received pilot channel signals separated into the first and second pilot channel signals since the received pilot channel signals received through the antenna of the mobile terminal are the combined first and second pilot channel signals. Consequently, these steps, such as generating two antennas symbol patterns and despreading the received signals, make the hardware structure of the mobile station very complicated. The integrators 512, 513, 514 and 515 must integrate 512 chips (that is, two times 256 chips), which corresponds to one symbol period, so that this limitation in designing circuits results in negative effects in power measurement in conditions such as fading, frequency error, or so on.